1. Field of the Invention
The present invention relates to a magnetic memory device having a magnetoresistive element including a magneto-sensitive layer whose magnetization direction changes according to an external magnetic field and recording/reading information by using a change in the magnetization direction of the magneto-sensitive layer and to a method of manufacturing the same.
2. Description of the Related Art
Conventionally, as general memories used for an information processor such as a computer or a communication device, volatile memories such as a DRAM (Dynamic Random Access Memory) and an SRAM (Static RAM) are used. The volatile memories have to be refreshed by always supplying current to hold stored information. When the power source is turned off, all of information is lost, so that a nonvolatile memory as means for recording information has to be provided in addition to the volatile memory. For example, a flash EEPROM, a magnetic hard disk drive, or the like is used.
In the nonvolatile memories, as the speed of information processing increases, increase in speed of an access is becoming an important subject. Further, as a portable information device is being rapidly spread and the performance is becoming higher, information device development aiming at so-called ubiquitous computing such that information processing can be performed everywhere at any time is rapidly being progressed. Development of a nonvolatile memory adapted to higher-speed processing as a key device of such information device development is in strong demand.
As a technique effective to increase the speed of the nonvolatile memory, a magnetic random access memory (hereinbelow, described as MRAM) is known in which magnetic memory elements each for storing information in accordance with the magnetization direction along the axis of easy magnetization of a ferromagnetic layer are arranged in a matrix. The MRAM stores information by using a combination of the magnetization directions in two ferromagnetic members. On the other hand, stored information is read by detecting a resistance change (that is, a change in current or voltage) which occurs between the case where the magnetization direction is parallel to a reference direction and the case where the magnetization direction is antiparallel to the reference direction. Since the MRAM operates with the principle, it is important that the resistance change ratio is as high as possible to perform stable writing and reading in the MRAM.
The MRAM currently used in practice utilizes the giant magneto-resistive (GMR) effect. The GMR effect is a phenomenon such that when two magnetic layers are disposed so that their axes of easy magnetization are parallel with each other, in the case where the magnetization directions of the layers are parallel to the axis of easy magnetization, the resistance value becomes the minimum. In the case where the magnetization directions are antiparallel with the axis of easy magnetization, the resistance value becomes the maximum. An MRAM using a GMR element capable of obtaining such a GMR effect (hereinbelow, described as GMR-MRAM) is disclosed in, for example, U.S. Pat. No. 5,343,422.
Recently, aiming at further improvement in storing speed, access speed, and the like, an MRAM having a TMR element using tunneling magneto-resistive effect (TMR) is proposed in place of the GMR-MRAM. The TMR effect is an effect such that the tunnel current passing through an insulating layer changes in accordance with relative angles of the magnetization directions of two ferromagnetic layers sandwiching a very-thin insulating layer (tunnel barrier layer). When the magnetization directions of the two ferromagnetic layers are parallel with each other, the resistance value becomes the minimum. On the contrary, when the magnetization directions are antiparallel to each other, the resistance value becomes the maximum. In the TMR-MRAM, when the TMR element has a configuration of, for example, “CoFe/aluminum oxide/CoFe”, the resistance change ratio is high as 40% and the resistance value is also large. Consequently, the TMR-MRAM can be easily matched with a semiconductor device such as an MOSFET. Therefore, the TMR-MRAM can easily obtain a higher output as compared with the GMR-MRAM, and improvement in storage capacity and access speed is expected. In the TMR-MRAM, a current magnetic field is generated by passing current to a conductor as a write line disposed near the TMR element. By using the current magnetic field, the magnetization direction of the magnetic layer in the TMR element is changed to a predetermined direction, thereby storing information. As a method of reading stored information, a method of passing current in the direction perpendicular to a tunnel barrier layer and detecting a resistance change in the TMR element is known. Such TMR-MRAM techniques disclosed in U.S. Pat. No. 5,629,922 and Japanese Patent Laid-open No. Hei 9-91949 are known.
Recently, higher packing density of a magnetic memory device is in increasing demand and, accordingly, reduction in the size of the TMR element is also required. As the TMR element is becoming finer, due to the influence of a demagnetizing field generated by magnetic poles at both ends of the TMR element, a large magnetic field is necessary to adjust the magnetization direction in a magnetic layer (free magnetization direction layer) for storing information to a predetermined direction, and write current required at the time of writing information is increasing. To address the problem, a magnetic memory cell having a structure in which a closed magnetic circuit is formed in cooperation with the free magnetization direction layer around a conductor (write line) near the TMR element is proposed (refer to, for example, Japanese Patent Laid-open No. 2001-273759). According to Japanese Patent Laid-open No. 2001-273759, the closed magnetic circuit is constructed by a free magnetization direction layer related to recording, so that the adverse influence exerted by the demagnetizing field can be avoided and a magnetic memory device of high packing density can be realized. Further, in this case, both of two write lines extend in the closed magnetic circuit, so that magnetization can be efficiently inverted.
However, in the magnetic memory device including the magnetic memory cell having the closed magnetic circuit structure as disclosed in Japanese Patent Laid-open No. 2001-273759, the structure is more complicated as compared with a magnetic memory device which does not have the closed magnetic circuit structure. At the time of manufacture, a larger number of processes is therefore necessary, so that reduction in the number of processes is demanded.